Srpenica · Soča Valley
The closest place to get in is Most Lipuc, a ten-minute walk from the apartment. Free, and nobody to book with.
It is cold: 8-12 °C even in August. Get in slowly, never jump into water you have not checked, and stay out after heavy rain when the level rises fast.
the warm river - the one to actually swim in
The Soča is spectacular and cold. The Nadiža is the opposite: shallow, slow and genuinely warm by comparison, which makes it the one to swim in rather than gasp in. Worth the drive on a hot afternoon, and much better with children.
the Soča beach, ten minutes on foot
There is a proper beach on the Soča here - shingle, shallow edges and deep green water under the footbridge. Ten minutes' walk from the door, free, and nobody to book with. Read the river safety notes on the Essentials page before you get in.